This invention relates generally to rolls utilized for pressure fusing toners at elevated temperatures, and more particularly to a roll which will prevent offsetting of toner onto the roll during the fusing operation.
In the process of xerography a light image of an original to be copied is typically recorded in the form of a latent electrostatic image upon a photosensitive member with subsequent rendering of the latent image visible by the application of electroscopic particles, commonly referred to as toner. The visual toner image can be either fixed directly upon the photosensitive member or transferred from the member to another support, such as a sheet of plain paper, with subsequent affixing of the image thereto. Toners are well known in the art and may be of various types.
In order to affix or fuse electroscopic toner material onto a support surface permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent into the fibers or pores of support members or otherwise upon the surface thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member. In both the xerographic as well as the electrographic recording arts, the use of thermal energy for fixing toner images onto a support member is old and well known.
Several approaches to thermal fusing of electroscopic toner images onto a support have been described in the prior art and include providing the concomitant application of heat and pressure as by a roll pair maintained in pressure contact, a flat or curved plate member in pressure contact with a roll, a belt member in pressure contact with a roll, and the like. Heat may be applied by heating one or both of the rolls, plate members or belt members. The fusing of the toner takes place when the proper combination of heat, pressure and contact time are provided, the balancing of these parameters being well known in the art and varying according to various factors which must be independently determined for each particular situation.
During operation of a fusing system of the type where there is a thermal fusing of electroscopic toner images onto a support in which at least one fuser member, such as a roll, plate or belt, is heated, the support member to which the toner images are electrostatically adhered, is moved through the nip formed between the members with the toner image pressure contacting the fuser roll thereby to effect heating of the toner images within the nip. By controlling the heat transfer to the toner, virtually no offset of the toner particles from the copy sheet to the fuser member is experienced under normal conditions. This is because the heat applied to the surface of the fuser member is insufficient to raise the temperature of the surface of the member above the "hot offset" temperature of the toner at which temperature the toner particles in the image areas of the toner liquify and cause a splitting in the molten toner resulting in "hot offset." Splitting occurs when the cohesive forces holding the viscous toner mass together is less than the adhesive forces tending to offset it to a contacting surface such as a fuser roll, fuser belt, or fuser plate.
Occasionally, however, toner particles will be offset to the fuser roll by an insufficient application of heat to the surface thereof (i.e., "cold" offsetting); by imperfection in the properties of the surface of the roll; by the toner particles insufficiently adhering to the copy sheet; by the electrostatic forces which normally hold them there; or by the reactivity of the toner material itself in those cases where the toner is of a reactive nature. In such a case, toner particles may be transferred to the surface of the fuser member with subsequent transfer to the backup member which provides provides pressure contact, during periods of time when no copy paper is in the nip.
One arrangement for minimizing the foregoing problems, particularly that which is commonly referred to as "offsetting," has been to provide a fuser member with an outer surface or covering of polytetrafluoroethylene, known by the tradename Teflon, to which a release agent such as silicone oil is applied. More recently, bare metal fuser members have been introduced for fusing or fixing the electroscopic toner materials to various surfaces. Various fluid polymer release materials which oxidize or which contain functional groups can be utilized to prevent "offsetting." Exemplary of such systems are disclosed in U.S. Pat. No. 3,937,637 and U.S. Pat. No. 3,918,804. Other release agents for bare metal fuser rolls are described in Belgium Pat. No. 831,662.
Typical materials heretofore proposed for bare metal fuser rolls includes anodized aluminum and alloys thereof, steel, stainless steel, nickel and alloys thereof, steel, stainless steel, nickel and alloys thereof, nickel plated copper, chrome plated copper and glass. Generally, copper is superior as a bare metal fuser member material because it has desirable release characteristics when used with certain release fluids. However, the life of bare copper fuser rolls is limited due to the softness and other properties of the copper. Accordingly, when bare metal copper rolls are used in a fuser system, excessive wear rates and the tendency of the bare metal copper roll to receive gouges, nicks and other deformities, necessitate the frequent replacement of the bare metal copper fuser rolls, resulting in uneconomical disadvantage because of replacement cost and machine down time.